Apparatus for the continuous on-site chemical reprocessing of ultrapure liquids

ABSTRACT

Method and apparatus for the distillation of liquids which is particularly suited for the removal of soluble impurities and insoluble and non-volatile particles of 10 microns to 0.2 micron or less in size. A substantially elongated distillation chamber having walls equipped with axially disposed concentric boiling rings spaced from the walls near the bottom, and a packing stop, packing redirector rings for condensed vapor, and a reflux condenser in the upper part of the distillation chamber provide, during distillation, a smooth convective upward flow of distilling liquid and vapor proximate the walls and boiling rings and a smooth convective downward flow of distilling liquid and vapor substantially centrally of the distillation chamber.

This application is a continuation-in-part of application Ser. No.07/183,089; filed Apr. 19, 1988 now U.S. Pat. No. 4,855,023, which is adivision of Ser. No. 06/915,776, Filed: Oct. 6, 1986, now U.S. Pat. No.4,828,660.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus for the continuouson-site purification of ultrapure liquids, especially liquids used in asemiconductor wafer cleaning process, such as ultrapure peroxydisulfuricacid and sulfuric acid solutions.

2. Description of the Prior Art

In the past, it has been common practice in industries requiringchemicals, especially ultrapure chemicals, to utilize such chemicalsuntil a certain degree of contamination was reached. At that point, itwas necessary to remove the contaminated chemicals from the processapparatus, clean the apparatus, and add new chemicals as needed.Contaminated chemicals were commonly disposed of by any convenientmeans. This has included legal and illegal dumping in land areas andoccasionally in waterways.

In the semiconductor industry it is important to remove all organic andinorganic particles from the surface of semiconductor wafers. This iscommonly done by immersion in an acid bath. A preferred acid bathconsists of an oxidant solution of sulfuric acid and eitherperoxydisulfate ion, which has the formula S₂ O₈ ⁻², or hydrogenperoxide and ultrapure water. The oxidant solution is commonly made bymixing together the oxidant and sulfuric acid. This combination producesa highly oxidizing compound which attacks carbon or other organicparticles on the surface of the wafers.

The wafers are commonly held in a cassette boat whereby they can becleaned by immersion into a tank containing the oxidant solution. Thetime for immersion is usually about ten to twenty minutes. Afterimmersion, the cassette boat containing the wafers is then washed inultrapure water. The purity of the water is determined by measuring theresistivity of the water.

In prior art processes very high purity sulfuric acid and H₂ O₂ or asource of peroxydisulfate are required. The bath temperature ismaintained at about 80° C.-150° C. In about one half hour, contaminationof the acid takes place with an increased concentration of particles. Atthis time, the acid is normally dumped and a new bath of high purityacid is added.

In recent years two developments have made this approach undesirable.The first of these has been the requirement of increasingly greaterpurity of chemicals, especially in industries such as the semiconductorand pharmaceutical industries. The second development has been anincrease in concern for the environmental effects of the dumping ofhazardous waste materials in the sewer lines, as well as on land.

With regard to the purity of chemicals, it is evident that the purity ofa liquid over a period of time is greater at the start of a process timeperiod than it is at the end of that time period. As greater purity hasbecome more and more important, it has become apparent that higherquality is produced using chemicals during the first part of the periodwhen purity is greater, than at the end of the tolerable processingperiod when contaminants have been able to build up in the chemicalliquid. As a consequence, in the specific case of the cleaning ofsemiconductor wafers using peroxydisulfuric acid, the wafers cleaned atthe beginning of the process period have a higher quality than thosewhich are cleaned at the end of the tolerable contaminant processingperiod.

With respect to the dumping of hazardous chemicals, public awarenesscoupled with recently passed hazardous waste chemical disposal laws,have made the disposal of hazardous chemicals extremely difficult aswell as costly.

In addition, the necessity of periodic replacement of chemically pureliquids represents an increased cost of materials, increased laborcosts, as well as a small but real risk of contamination or hazard tothe personnel involved. Finally, there is the cost involved in shuttingdown a process for whatever time is required to replace the chemicals.

In addition, any time chemicals are stored or transferred, impuritiesare introduced which are intolerable for ultrapure requirements. Forexample, stabilizers often must be added to prevent decomposition ofunstable compounds. Also, reaction with the containers during storageand transfer, although slight in most cases, often produces acontamination level in such liquids which is intolerable for ultrapureprocess requirements.

Manufacturing space available for semiconductor manufacturing is oftenlimited and expensive. Moreover, local government codes often limit theamount of sulfuric or other acid which can be inventoried in any onecontainer.

In the case of semiconductor wafer cleaning various chemicals can beused. One process utilizes hydrogen peroxide which must be shipped withstabilizers in order to prevent spontaneous decomposition. Thestabilizers which are required to be used introduce impurities whichwill ultimately contaminate the wafers during the cleaning process.

Another process utilizes potassium or ammonium peroxydisulfate.Potassium peroxydisulfate commonly contains metal ions as impuritieswhich produces a known problem with integrated circuits, particularlyMOS circuits.

While ammonium peroxydisulfate could theoretically be made quite pure,such purity levels are not available on an economically attractivebasis.

In light of the above difficulties in requirements for the use ofultrapure chemicals and the subsequent contamination and disposalrequirements, it is desirable to provide a method and apparatus capableof maintaining purity of the ultrapure liquid throughout the course ofthe reaction which will avoid contamination buildup. In addition, it isdesirable to provide a process and apparatus which avoid the need forthe disposal of large amounts of hazardous chemicals. Finally, it isdesirable to provide a process and apparatus which reduce processingcosts by reducing the amount of chemicals required, reducing the numberof personnel involved, increasing the safety of the personnel involved,and eliminating the frequent requirements for shutdown of the processfor purposes of renewing ultrapure liquids.

One object of the invention is to provide a novel method and apparatusfor distillation of liquids to remove soluble impurities and insolubleparticles, especially particles having a diameter of less than 10microns.

Prior art glass distillation columns are commonly spherical in shape,having a diameter of about two feet and a capacity of approximately 30gallons. The output is about 600 ml per minute of sulfuric acid. Theliquid boiling volume in the boiling pot is about 15 gallons providing aliquid/vapor boiling surface of about 3 square feet at the boilingsurface and a heat transfer area of about 6.5 square feet.

The distillation column of the invention is preferably a cylinder ofabout 6 feet in overall height including a cylindrical bottom having adiameter of about 8 inches and a boiling liquid height of about 2 feetfor the same throughput of 600 ml per minute. The boiling volume isabout 5 gallons which is one-third of the prior art distillation column.However, the heat transfer area is about 5 square feet with a boilingsurface area of about 1/3 square foot. This design saves about 1 foot infurnace diameter at the base when the heating element is included andreduces the inventory of boiling acid by one-third. This is aparticularly desirable safety feature. Other features of this inventionallow for improved system performance even with a smaller heat transferarea.

Nucleation sites are required to avoid bumping which can be verydangerous when acid is distilled since the acid can be forced out of thedistillation column. Metal boilers are inherently rough which providesnucleation sites for boiling. However, metal boilers are unsuitable forthe invention process since they introduce metallic contaminants. Mostdistillation in glass boilers generally utilize boiling chips in theform of rough ceramic inert granules to provide nucleation sites sincethe glass surface is extremely smooth and does not inherently providenucleation sites. It was found that use of these chips wasunsatisfactory as the chips scoured the glass over time and createdparticles which contaminated the sulfuric acid.

This problem is overcome by the distillation column of the invention. Aparticularly novel feature of the distillation column is the inclusionof porous fused quartz or other glass boiling rings which are spacedfrom and fused at selected points to the distillation column walls.

The preferred material for these boiling rings is composed of QuartzScientific "TPL"™ in the form of fused quartz which is formed by fusingsilicon dioxide powder with heating from one side. This results in aninner surface which is smooth and glassy and an outer surface which ishighly textured. The textured surface provides interstices for vapornucleation. The resulting piece is sawed into rings and mounted in thedistillation column spaced from the wall.

It was found that a particularly novel advantage of locating boilingrings spaced from the distillation column wall is that a novel smoothconvective boiling pattern is created. Boiling with bubble and vaporformation takes place in the highest heat zone which is nearest thecolumn walls. This is believed to cause a predominant circulationpattern wherein the vapor rises smoothly up the interior wall surfacesof the distillation column on both surfaces of the boiling rings. Theresultant smooth convective flow reduces entrainment of contaminants,particularly soluble and insoluble particles of less than about 10microns in size.

At the same time with the distillation column of the invention, coolerunvaporized liquid also rises to the surface but returns to the bottomof the column in the relatively cooler center portion. Not only doesthis feature insure smooth, energy efficient boiling but there is also areduction in the boiler wall temperatures.

Wall temperatures are a direct function of the efficiency of heattransfer from the heating zone or furnace to the liquid since boilingcools the walls by absorbing the energy. The distillation column of theinvention provides a higher efficiency of heat transfer than does theprior art method by causing boiling to take place at the wall due to thespaced boiling rings. This advantage not only reduces the required heattransfer area and energy costs but also provides a smooth convectiveboiling pattern which reduces entrainment of particles.

One of the most advantageous features of the distillation column of theinvention is the ability to remove both soluble impurities and insolublesolid particles of less than about 10 microns.

In order to produce ultrapure liquids, it is particularly necessary toremove particles, particularly insoluble small particles in the range ofone micron and smaller sized particles. When distillation is used as amethod for purification, there are problems associated with the removalof small particles. This is due to the fact that the small particle, forexample a particle of less than 10 microns in size, can be expected tobe carried over by the vapor during distillation since such particleswould have a mass/cross sectional area that would prevent gravityseparation from the vapor flow.

Not only are the particles expected to be released into the vapor streamduring the agitation caused by boiling, but also it would be expectedthat as the bubble forms the particle or particles would be carried tothe surface on the bubble liquid interface and ejected into the vaporduring the distillation. Thus, it would be expected that all smallparticles would be entrained in the rising vapor stream to be carriedover to the product reservoir.

It is an object of the invention to provide a distillation apparatus andmethod which clearly separates small particles, especially particles ofless than 10 microns, from the vapor distillate in contrast to theexpected entrainment thereof.

A combination of features of the distillation column of the inventionhave made possible the effective removal of insoluble impurities andnon-volatile solid particles having a size down to the limits of liquidparticle counters, i.e. 0.2 microns.

One novel feature of the distillation column of the invention which isbelieved to contribute to particle removal includes in particular theuse and location of the boiling rings which optimize smooth boiling atthe liquid/vapor interface and provide efficient heat transfer resultingin fewer particles being expelled into the vapor stream. The resultingsmooth convective flow of vapor bubbles upwardly along the walls and thedownward flow of cooler liquid substantially centrally of thedistillation column is also believed to be novel.

Another feature of the distillation column of the invention which isbelieved to contribute to particle removal is the provision of a packedcolumn as an efficient counter current particle scrubber which enablesthe reflux stream to continually wash the rising vapor and particlesback down the column.

Another feature of the distillation column of the invention is theprovision of redirector rings which together with the packing causecomingling of the acid and vapor to effect further scrubbing of thevapor/reflux liquid streams. The redirector rings also direct condensedvapor to fall within the central area of the distillation column.

Still another novel feature of the distillation column of the inventionwhich is believed to contribute to particle removal is the provision ofa low net vapor velocity by sizing the distillation column with arelatively large diameter in relation to the throughput. This permitsincreased dwell time for rising vapor to be scrubbed by counter currentdownward flow of condensed liquid.

The use of glass in apparatus for distillation gives rise to problemswhich are unique to such use. These include among others, thebrittleness of glass, and in the case of distillation of highlycorrosive liquids the consequent risks involved if glass is broken.Moreover, glass is not flexible so special care and design are needed toprovide joints which will flex and not break or leak upon expansion ofglass during the relatively high distillation temperatures, for example300° C.

The distillation apparatus of the invention provides a design whichovercomes the above problems unique to glass and at the same time is ofa relatively small size to limit the quantity of corrosive liquid whichmust be handled. This reduces the risk inherent in the event of breakageand at the same time operates within existing city code limitations withrespect to the total amount of corrosive material which can beinventoried.

SUMMARY OF THE INVENTION

The novel process and apparatus provided by this invention overcomes thedeficiencies of the prior art by continuously withdrawing usedacid/oxidant solution from the process stream, subjecting it topurification techniques, and then reintroducing it to the process streamto maintain a constant ultrapure liquid concentration having a knownpurity.

With respect to the production of ultrapure liquids for semiconductorwafer cleaning, the process begins with an oxidant solution of ultrapuresulfuric acid and peroxydisulfuric acid in ultrapure water. During thecourse of the acid cleaning, the peroxydisulfuric acid graduallydegenerates or degrades to sulfuric acid and water. This degradedoxidant solution comprising sulfuric acid and water is continuouslywithdrawn from the process stream and repurified. Peroxydisulfuric acidis generated in-situ from repurified sulfuric acid via the action of anelectrochemical cell. The regenerated oxidant solution together withrepurified sulfuric acid are continuously added to the wafer cleaningbath to maintain a constant volume and concentration of oxidantsolution.

The purification process is continuous, permitting three to four acidchanges per cleaning bath. This is based on a 60 liter per hour flowrate, a bath size of 4 liters, and a cleaning time of 10 minutes. Thisamounts to approximately 300 cc to 500 cc of clean acid per wafer by thenew process compared to 10 cc to 50 cc of clean acid per wafer in thecurrent stagnant cleaning bath prior art processes. Since the solutionis reprocessed, spray processes maybe optimized by increasing the volumeof acid per wafer. The current art of spray and discard limits the acidvolume for economic reasons.

Since the chemicals used are continuously purified, only small amountsof contaminated liquids require disposal. In addition, only smallamounts of makeup acid or makeup ultrapure liquids are required to keepthe volume constant.

Since no stabilizers are used and only occasional transfer and storagecontainers are necessary, the introduction of impurities is minimized.

Standard commercially available 90%-98% sulfuric acid contains10-100,000 particles/cc of acid. These particles have a size of 1-15microns. Trace impurities primarily in the form of cations are alsopresent in the amount of 10 PPB. Both particles and trace elementsconstitute undesired impurities on semiconductor wafers. The process ofthe invention reduces particle concentrations of 1 micron size andgreater to <5/cc and trace impurities <10 PPB.

Particles, particularly particles less than 10 microns, are removed by anovel distillation process and apparatus whereby boiling nucleationsites are provided near the walls of the distillation column. Thus, heatapplied to the walls is efficiently transferred to the liquid causingsmooth boiling and upward convection of vapor bubbles along the interiorwalls. At the same time the distillation column is provided with meansto direct condensed vapor and to distribute the liquid more evenlythrough the packing.

A particular feature of the process avoids the pumping of the mainstream of reprocessed ultrapure liquids, avoiding the introduction ofpump induced contamination inherent in mechanical pumps. Pumping isemployed during parts of the reprocessing process for only about 4% ofthe reprocessing stream. Non-contaminating pumping and delivery systemscan also be used.

In addition, since the process is continuous, it is possible tocontinuously monitor the purity of the process chemicals in line. Thisis in contrast to prior art procedures which permit only the monitoringof lot samples of incoming chemicals since analysis of the chemicals atthe use station is impractical. Since only lot samples are used, thereis always the possibility that a single batch of incoming chemicals iscontaminated which would not be evident from the prior art method oftesting only lot samples. This can result in the contamination of wafersduring the wafer cleaning process which will not be immediately evident.The invention method assures purity throughout the process.

While the invention is particularly described with respect to thepurification of ultrapure liquids, particularly an oxidant solution ofsulfuric acid, peroxydisulfuric acid, and ultrapure water for use insemiconductor wafer cleaning processes, it is contemplated that theinvention is applicable to the continuous on-site purification of otherchemicals especially sulfuric acid to ultrapure standards. Suchpurification is intended to include but is not limited to, for example,mineral acids and solvents. These chemicals might be used but are notlimited to such industries as the semiconductor manufacturing industry,pharmaceutical manufacturing, pc board manufacturing, magnetic tape ordisk manufacture, laser disk manufacture, metal finishing industries, orany other application which requires purified chemicals.

Similarly, the exact process for the continuous on-site chemicalreprocessing of the ultrapure liquids will involve various chemicalprocess technologies, depending on the nature of the ultrapure liquid tobe continuously repurified. Such process technologies can include, butare not limited to distillation, such as atmospheric and vacuumdistillation, electrochemical regeneration, electrodialysis, filtration,centrifuging, ion exchange, gettering, sublimation, and adsorption.

The invention is specifically described with respect to the integrationof these processes and their process conditions to the continuousregeneration and repurification of an oxidant solution of sulfuric acid,peroxydisulfuric acid, and ultrapure water for use in cleaningsemiconductor wafers.

A system comprising apparatus to conduct the repurification process ofthe invention is also provided. This system includes novel distillationapparatus whereby contaminants in the form of soluble impurities andinsoluble particles can be removed, a novel water/acid stripper whichseparates acid from water and volatile components, and a novelelectrical cell where chemicals are generated for the wafer cleaningprocess, and an integrated automatic chemical make-up.

The novel distillation apparatus is capable of removing particles fromless than 10 microns to below 0.3 microns or less in size. Moreover, thedistillation column is sized especially tall so that the product can begravity fed to the use point. This avoids the need for a pump whichwould be a source of contaminants. Operating costs are also reducedthereby.

A computer controlled valve can be used to automatically quench boilingif a rapid system shutdown is required. This is achieved by discharge ofcold liquid from cooled product inventory. Thus, a significant safetyfeature is provided in case a rapid shutdown is required.

The efficiency of the novel distillation column coupled with the lowimpurity feed stream avoids the need for continuous withdrawal of thedirty acid in the bottom of the distillation column.

The distillation column of the invention includes porous fused quartzboiling rings which are spaced from and fused at selected points to thedistillation column walls. The provision of attaching the boiling ringsin a location spaced from the distillation column walls allows boilingto take place in the highest heat zone and causes the vapor to risesmoothly up both sides of the boiling rings near the interior surface ofthe distillation column while the cooler unvaporized liquid also risesto the surface but returns to the bottom of the column in the relativelycooler center portion. This feature insures smooth, energy efficientboiling and a reduction in the boiler wall temperatures to provide ahigher efficiency of heat transfer due to the spaced boiling rings.

The distillation column of the invention includes a packed column andredirector rings as an efficient counter current particle scrubber whichenables a reflux stream to continually wash the particles back down thecolumn. A reflux condenser head or a total condenser which provides areturn overflow of condensed product causes a counter current flow ofvapor and liquid through the packing and redirector rings.

Finally, the distillation column of the invention provides a low netvapor velocity for small particle removal since the distillation columnis sized with a relatively large diameter in relation to the throughput.

As used herein and in the appended claims, the term "nucleation sites"refers to a crevice, pore, or interstice where a gas bubble formingwithin a liquid can nucleate and grow to a size permitting bubbling tothe liquid surface.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the system of the invention;

FIG. 2 shows a detailed schematic view of the stripper 1 shown in theblock diagram of FIG. 1;

FIG. 3 shows an enlarged view of the inert gas bubbler manifold of FIG.2;

FIG. 4 shows a detailed schematic view of the distillation column 2 andcondenser and cooler 3 shown in the block diagram of FIG. 1;

FIG. 5 shows an enlarged view of the packing stop shown in FIG. 4;

FIG. 6 shows a detailed schematic view of the cooler 5 shown in FIG. 1;

FIG. 7 shows a detailed schematic view of the diluter 7 and water cooler9 shown in the block diagram of FIG. 1;

FIG. 8 shows a schematic sectional view of the wafer cleaning processstation 11;

FIG. 9 shows a schematic top plan view of the wafer cleaning bath ofFIG. 8;

FIG. 10 shows an enlarged detailed schematic view of the wafer bath ofFIG. 8;

FIG. 11 shows a schematic representation of the electrical cell shown at10 in the block diagram of FIG. 1;

FIG. 12 shows a sectional view of the electrical cell 10;

FIG. 13 shows an enlarged perspective view of the water cooled anode ofFIG. 12;

FIG. 14 shows an enlarged perspective view of the cathode of FIG. 12;

FIG. 15 shows a perspective view of the apparatus of the invention, aportion of which is enclosed in a housing;

FIG. 16 shows a detailed view of the apparatus enclosed in a housing;

FIG. 17 shows a top plan view of the apparatus of FIG. 16;

FIG. 18 shows a perspective view of one of the redirector rings emplacedin the distillation column of FIG. 2;

FIG. 19 shows a detailed schematic view of another embodiment of thedistillation column; and,

FIG. 20 shows a schematic representation of the smooth, convective flowpattern of rising vapor and falling reflux liquid which takes placeduring distillation according to the invention method using thedistillation apparatus of FIGS. 4 and 19.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 there is shown a block diagram of the process ofthe invention. The process is continuous, and, thus, for convenience adescription of the process will begin at the semiconductor wafer cleanershown in Block 11.

An oxidant solution bath is first prepared and is comprised of at least0.05M H₂ S₂ O₈, 92% by weight of H₂ SO₄, and the balance ultrapurewater. The temperature is typically about 80° C. to about 150° C. Here,cassettes of wafers are subject to acid cleaning to remove anyimpurities therefrom. This is normally accomplished by immersion in thebath, but other means such as a spray can also be used. The cleaning canbe augmented by means of ultrasound and megasound if desired (10⁺³ -10⁺⁶Hz).

During the wafer cleaning process, the oxidant solution which is used toclean the semiconductor wafers is continuously withdrawn and circulatedthroughout the system. The oxidant solution is withdrawn continuously bygravity overflow to a surge tank 12 where it is first directed to afluoride removal column or tank 14. The purpose of the fluoride removalcolumn 14 is to remove any fluoride ions which are present as acontaminant on the wafers as a result of previous process steps.

It is desirable to remove the fluoride ions for two reasons. The firstis that it is an undesired impurity in the acid solution, and secondly,it is very destructive to many types of materials, especially fusedquartz glass which is the preferred material for this apparatus.

From the fluoride removal column 14 the oxidant solution containing acidand water is directed to the stripper shown in block 1. Here, thesolution is heated to cause the water to vaporize. At the same time aninert gas such as nitrogen is bubbled through the solution to removevolatile impurities and water vapor. The water vapor and entrainedimpurities escape to condenser 13 and any acid vapor which is alsopassed into condenser 13 is condensed and returned to the surge tank 12.At the same time, the water vapor and impurities leave the system via anexhaust duct.

The acid stripped of the water in the stripper 1 is then directed todistillation column 2 where it is heated to a very high temperature,causing it to vaporize. This produces a very pure vapor separated fromparticles and other contaminants since the distillation column includesspecial boiling rings, packing materials, redirector rings as well as areflux head. The acid vapor then passes from the distillation column 2to the condenser and cooler 3 where it is condensed and cooled prior tobeing directed to a stream splitter 4.

The stream splitter 4 sends a major portion amounting to approximately90% to 98% of the purified acid stream directly to the semiconductorwafer cleaner 11. The remaining portion of the stream amounting to about2% to about 10% is directed to a water cooler 5 prior to being directedto a particle counter 6. At the particle counter 6, the purity of theacid is measured for quality control purposes.

The acid from the particle counter 6 is then directed by means of ametering pump 17 to a diluter shown in block 7. Here it is mixed withabout 40% to about 70%, and most preferably 50% by weight of ultrapurewater.

The heat of solution causes the temperature of this mixture to rise. Itis cooled primarily by a water jacket within diluter 7 which reduces thetemperature to about 15° C. to abut 25° C. before being directed toelectrical cell 10 (E-cell). A small portion of the liquid stream isdiverted after dilution to an atomic absorption spectrometer 16 fortrace analysis.

At E-cell 10, the preferably 50% by weight sulfuric acid/water solutionis changed to an oxidant solution of at least 0.5M peroxydisulfuric acidand 50% by weight sulfuric acid, with the balance ultrapure water. Theoxidant solution is then sent to the semiconductor wafer cleaner 11where it is mixed with main stream incoming purified sulfuric acid fromthe condenser and cooler 3. Any excess or overflow is directed to thesurge tank 12.

At the semiconductor wafer cleaning station 11, the process loop iscomplete and the reprocessing proceeds continuously.

The relatively unstable nature of the oxidant solution producedaccording to the process of the invention precludes its storage for longperiods of time. The fact that the oxidant solution is freshly generatedcontinuously constitutes a particular feature of the invention andconstitutes a significant step over prior art processes.

While the process is particularly designed for continuous reprocessingof the oxidant solution, certainly the process can be applied forcontinuous batch repurification. In this instance, oxidant solutionwithdrawn from the wafer cleaning station could be held in at least onereservoir. If at least two reservoirs are used, one could be repurifiedaccording to the invention process while the other reservoir could befilling with degraded oxidant. The repurification process would thenalternate between the two or more reservoirs to always provide a sourceof pure oxidant solution.

Each of the blocks shown in FIG. 1 are discussed in greater detailbelow.

THE SURGE TANK

During the semiconductor wafer cleaning process, the preferred oxidantsolution comprised of at least about 0.05M peroxydisulfuric acid, about92% by weight sulfuric acid, and the balance ultrapure water becomesdegraded during the process. The wafer bath continuously overflows tothe surge tank 12. In addition, any overflow from the condenser andcooler 3, the stripper condenser 13, and the anolyte reservoir 120 orcatholyte reservoir 118 of the E-cell 10 is directed to the surge tank12. Moreover, any needed make up sulfuric acid is added to the processstream from the acid make up tank 18 at this point. This is necessary tokeep the volume of the process stream constant since small amounts ofthe stream are continuously being removed in the form of waste from thestripper 1, the distillation column 2, and dragout from the bath 11.

A pump 15 continuously pumps acid solution from the surge tank 12 to thefluoride removal tank 14. This can be any standard pump, preferably a"Teflon" (TM) pump, which is made of materials resistant to acidcorrosion. "Teflon" (TM) (polytetrafluoroethylene) is inert to thecleaning solution and for this reason is preferred.

FLUORIDE REMOVAL COLUMN

The fluoride removal tank or column 14 is not detailed in the drawings.It can be any standard chamber in the form of a tank or an elongatedcolumn filled with activated alumina (aluminum oxide) beads or otherchemical reactive with F⁻ ions. The F⁻ ions are undesired in thereprocessing method due to their reactivity with fused quartz glass ofwhich much of the apparatus is composed. The fluoride removal tank orcolumn 14 has an inlet and an outlet for the passage of acid in and outof the tank. From the fluoride removal tank or column 14, the oxidantacid/water stream is .passed to the stripper shown in block 1 of FIG. 1.

THE STRIPPER

The stripper can be seen in greater detail in FIG. 2. As shown, aplurality of heating coils 20 in an insulated block 22 surround a largediameter vessel or fused quartz glass tube 24. Within the fused quartzglass tube 24 are a number of fused quartz glass inlet tubes. Theoxidant acid/water solution is introduced into the bottom of the tube 24by means of inlet tube 26, At the same time, an inert gas such asnitrogen gas is introduced through a fused quartz glass tube 28 whichenters near the top of the tube 24 and extends downwardly into the tube24 to a bubble manifold 30.

The manifold 30 is shown in detail in FIG. 3. As shown, the inlet tube28 for the gas terminates in the manifold 30 which is in the form of atubular circle having radial cross-members 31. A plurality ofperforations 33 permit the gas bubbles to escape into the solution.

A temperature probe can be inserted into a closed fused quartz glasstube 32 to permit the measurement of the temperature within the tube 24without invading the interior contents of the stripper 1. Another tube34 permits the removal of the stripped acid from the stripper tube 24.The tube 24 also includes an outlet passage 36 near the top which opensinto a condenser tube 38 corresponding to block 13 of FIG. 1. Thecondenser tube 38 has an outlet passage 40 for the escape of water vaporand nitrogen gas and an exit tube 42 at the bottom thereof to drain awaycondensed acid.

The condenser tube 38 contains a coiled spiral tube 44 which is sealedwith respect to the interior of the condenser tube 38. Cooling fluid,preferably silicon oil or water, is circulated through coils 44 at atemperature of about 33° C. to about 80° C. In addition to the spiraltube 44, condenser tube 38 is packed with Raschig rings 39 which are 1/4inch diameter fused quartz glass tubing which has been chopped into 1/4inch lengths.

In operation, the oxidant acid/water solution at a temperature of about100° C. continuously arriving from the fluoride removal tank 14 entersthe quartz tube 24 by means of inlet tube 26. At the same time, nitrogenis continuously bubbled down through tube 28 to manifold 30 where thebubbles rise through the acid/water liquid contained therein.

The heating coils 20 in the insulated block 22 which surround the fusedquartz glass tube 24 continuously heat the oxidant acid/water solutionto approximately 280° C. which is below the boiling point of the acid.This heat and the partial pressure of nitrogen causes the water and asmall portion of the acid to vaporize.

At the same time, the bubbles of nitrogen gas which pass upwardlythrough the oxidant acid/water solution attract molecules of water vaporand low boiling compounds such as CO₂ to the bubble surfaces. Thesevolatile impurities and minimal acid vapor are then continuously carriedoff at the top of the tube 24 by the gas bubbles which upon penetratingthe surface form a fine mist.

This mist contains carbon dioxide, water vapor and other volatilecomponents which are continuously removed from the acid solution. Thebubbles then, carry these water vapor molecules as they rise through theoxidant acid/water solution and escape through passage 36 to condenser38. The remaining liquid contains more concentrated acid. In thismanner, most of the water is effectively stripped from the oxidantacid/water solution originally introduced into the tube 24.

In the condenser 38, the water vapor and the nitrogen gas rise andescape through passage 40. At the same time, cooling fluid such assilicone oil or water which is circulated through the sealed tubingcondenser coils 44 within the condenser 38 together with the packing ofthe Raschig rings 39 cools any acid vapor to a point below its boilingtemperature, causing it to condense and pass out of the condenser 38through drain 42 which is then directed to surge tank 12. The condensercoils are kept at a temperature which is greater than the dew point ofwater and less than the boiling point of water to insure that the waterwill escape in the form of a vapor and the acid vapor will be condensedand returned to the surge tank 12. Without the Raschig rings 39 and thecondenser coils 44, acid vapor would exit with the water vapor causingan overall loss of acid.

The cooling fluid such as water or silicone oil which is circulatedthrough tube 44 within the condenser tube 38 is cooled in a heatexchange unit not shown or by any other suitable means. In mostinstances this is practically accomplished by means of ambienttemperature city water. The same method is used for cooling oil andcooling water which are circulated for cooling purposes in other partsof the apparatus as subsequently described.

The nature of the cooling fluid is not critical and can comprise amongothers water or silicone oil. Preferably the cooling oil is a siliconeoil which has a boiling point between 400° C.-500° C. Such a hightemperature boiling point oil is necessary to be able to cool sulfuricacid which has a boiling point of 338° C.

Although nitrogen gas is preferred for use in the stripper 1, otherinert, clean, dry purified gases can be used in place of nitrogen. Suchinert gases include but are not limited to air, helium, neon and argon.

At the stripper 1, the acid which has been stripped of the water exitscontinuously from the tube 24 through exit tube 34. It is then directedto the distillation column 2 and cooler and condenser 3 of FIG. 1 whichare detailed in FIG. 4.

THE DISTILLATION COLUMN

As shown in FIG. 4, the distillation column 2 is comprised of a largediameter fused quartz glass distillation tube 46. The distillationcolumn 46 is surrounded at its base by heating coils 82 disposed in aninsulated block 84. This provides the necessary heat to the distillationcolumn 46.

In order to aid in the boiling process, a series of "snoball" quartzrings 50 are disposed annularly approximately 1/4 to about 1/2 inch fromthe wall of the fused quartz glass distillation tube 46 by attachment toseveral fused quartz glass support rods 52. The rods 52 are disposedvertically around the interior peripheral surface of the fused quartzglass tube 46 for purposes of holding the quartz rings 50.

The quartz rings 50 are preferably comprised of a special type of quartzcalled "snoball" quartz which refers to its appearance. Initially the"snoball" quartz is in the form of small beads of quartz which have afrosted appearance. The rings 50 are formed of a plurality of thesebeads which have been only partially melted together. The result is aperforated structure having a plurality of interstices within the ringsto act as boiling sites for the boiling acid. A preferred material forthe quartz boiling rings 50 is Quartz scientific "TPL"™.

The quartz rings are very important to the distillation column to aid inthe boiling of the acid. Commonly used boiling chips were initiallytried and found to be less satisfactory. The boiling chips moved aroundso much in the column that they resulted in a scouring effect on thecolumn surfaces. This is undesirable since it can produce contaminantsfrom the fused quartz glass. Other nucleation enhancement materials suchas sintered glass frits can also be used.

Another major advantage provided by the boiling rings 50 is that thereis more efficient heat transfer from the wall to the liquid. The boilingrings 50 are placed close to the walls so that the boiling of the liquidacid occurs near the source of heat. Bubbles of vapor form within theinterstices of the boiling rings and the bubbles rise on both sides ofthe rings. This causes a novel convective flow to occur which isillustrated schematically in FIG. 20.

As the boiling liquid and vapor rise along the walls of the distillationcolumn, the cooler liquid flows downwardly in the central area of thedistillation column. Thus, there is provided smooth convective flow andsmooth boiling in a novel manner.

The acid enters the fused quartz glass distillation tube 46 at opening54 where it is introduced near the vicinity of the quartz rings 50. Asmaller diameter tube 48 extending from one side of the tube 46 near thebottom is used for purposes of removing acid constituting waste.

Above the quartz rings 50 is a grid or packing stop 56. As detailed inFIG. 5, the grid 56 is in the form of a cross hatched fused quartz glasscircular disk which is attached to the walls of the quartz tube 46. Thegrid 56 extends across the tube 46 in a direction perpendicular to thecentral axis of the tube 46.

Within the quartz tube 46 above the grid or packing stop 56 are axiallydisposed redirector rings 57 in a form having a generally truncatedfunnel shape with a large delivery opening as shown in FIG. 18. Alsoabove the packing stop 56 and within and surrounding the redirectorrings 57 there is disposed packing comprised of Raschig rings 58 whichextend nearly to the top.

Raschig rings are comprised of 1/4 inch diameter fused quartz glasstubing which has been chopped into 1/4 inch lengths across the centralaxis. Other types of packing can be used in place of the Raschig ringsincluding among others Lessing rings, Berl saddles, partition rings,spiral rings and single and multiple turned helices of thin quartzglass. The packing is preferably of fused quartz glass. Quartz marblescould be used in place of the Raschig rings 58, if desired, but they areless preferred.

The purpose of the Raschig rings 58 and redirector rings 57 are to causethe considerable refluxing of the acid during the distillation process.The redirector rings are also important in directing the condensing acidvapor to the central portion of the distillation column. This furtherdrives the cooler liquid acid from the center to the walls where boilingtakes place in a smooth convective manner.

The refluxing of the acid is further enhanced by means of a refluxcondenser head 60 located at the top of the quartz distillation column46. It is comprised of a spiral tube 61 which is sealed with respect tothe interior of distillation column 46.

The spiral tube 61 contains cooling a cooling fluid such as water or anoil, preferably silicone oil, and is located in the vicinity of theoutlet passage 62 which leads to cooler and condenser 3. The coolingfluid such as water or oil has the effect of condensing acid vapor whichreaches it so that there is a continuous scrubbing action of the acid,so that only the purest acid vapor manages to escape through passage 62to the cooler and condenser 3 leaving behind particles and othercontaminants.

The effect of the reflux condenser 3, the redirector rings 57, andRaschig rings 58 within the column 46 as shown is to removecontaminants, especially particles. For every 4000 particles presentprior to distillation there will be only 1 particle present afterdistillation. Thus, the distillation process removes not only solublecontaminants but insoluble particles as well from the sulfuric acid.This represents a significant and novel step in the art. Thus, thedistillation column itself is a novel feature of the invention.

The cooler and condenser 3 shown in FIG. 4 is comprised of a largediameter fused quartz glass tube 64 disposed horizontally and having twosubstantially parallel spiral sealed tubes 66 and 68 disposed therein,although more tubes can be used as needed to provide increased coolingsurface area. Each of the tubes 66 and 68 contain circulating coolingfluid such as water or oil which is pumped through the tubes from a heatexchanger not shown as described for the stripper condenser.

The upper cooling coil 66 acts to condense the acid vapor as it reachesthe cooler and condenser 3. The lower disposed cooling coil 68 acts tocool the acid liquid as it is condensed so that it is further cooledprior to its exit from outlet 70. Upon exiting through outlet 70 theliquid acid is directed to stream splitter 4 as indicated in the blockdiagram of FIG. 1.

The quartz tube 64 also contains a vent 72 in the form of a smalldiameter tube 72 which extends partially into the quartz tube 64 in onedirection and in the other direction is in communication with acondenser 74. The condenser 74 communicates with a packed column 76. Thecondenser 74 contains a sealed spiral coil of fused quartz glass tubing78 through which cooling water is circulated for purposes of cooling anyvapor which escapes into the condenser 74 through vent 72. This vapor isfurther condensed by means of passage through packed column 76 which iscomprised of a fused quartz glass tube filled with a packing material80. From the packed tube 76 any condensed acid liquid is directed to thesurge tank 12 for recirculation through the system.

Another preferred embodiment of the distillation column of the inventionis shown in FIG. 19. As shown, the elongated distillation column 400similar to the distillation column 2 is comprised of a large diameterfused quartz glass distillation tube 400. Within the lower portion ofthe distillation column 400 are disposed a plurality of "snoball" quartz"preferably TPL"™ quartz boiling rings 50 which are disposed annularlyand spaced about one half inch from the wall of the quartz glassdistillation column 400. The quartz boiling rings 50 are attached toseveral fused quartz glass support rods 52 which are disposed verticallyaround the interior peripheral surface of the fused quartz glassdistillation column 400. The quartz rings 50 are identical with thequartz rings 50 shown in FIG. 4 and described above.

The distillation column 400 is disposed within an insulated block 84.Surrounding the lower vertical portion of the distillation column 400are heating coils 402. It is preferred that the heating coils 402 bedisposed substantially coextensively with the boiling rings 50 in orderto confine boiling to the vicinity of the walls and avoid boiling at thebottom of the column 400.

Acid to be distilled is introduced through opening 54, preferably belowthe liquid level and within the vicinity of the quartz boiling rings.

Disposed above the boiling rings 50 and above the liquid level withincolumn 400 is a packing stop 56 identical with that shown in column 46of FIG. 4 and detailed in FIG. 5.

As in the distillation column of FIG. 4, there are a plurality ofaxially disposed redirector rings 57 which are disposed above the gridpacking stop 56. The redirector rings 57 have a generally truncatedfunnel shape oriented with the large opening facing upwardly and thesmaller opening oriented downwardly exactly as shown in FIG. 18.

Above the packing stop 56 and within and surrounding the redirectorrings 57 there is disposed packing in the form of Raschig rings 58 whichextend near the top of the column 400. These Raschig rings are identicalwith the Raschig rings which are used in the distillation column of FIG.4.

In this embodiment of the distillation column there is no provision forthe removal of waste acid. It has been found that it is not necessary toremove waste acid since the distillation column can be operated as longas six months to a year without the need for the removal of bottomsproduct. This is due to the efficiency in removing soluble impuritiesand insoluble particles. The actual period of operation before removalof waste will depend on the process for which the purified acid is beingused.

The distillation column 400 is provided with a ball joint at the topincluding an open rounded neck 408 into which is inserted a ball 410having a groove therein not shown for seating of a Teflon O-ring 416.The ball 410 is attached to a condenser 464 by means of a quartz tube418.

The quartz tube 418 is disposed at a downward angle from the condenser464. Vapor which condenses within condenser 464 fills the base of thecondenser. When the level of condensed cooled liquid within condenser464 reaches a certain height, the liquid overflows the point 462 wherethe tube 418 is attached to condenser 464 which acts as a weir. Thiscooled liquid acid falls back down the tube 418 to provide a refluxstream which washes the rising vapor within the distillation column 400.This combined reflux action is particularly effective in removingparticles and other contaminants in conjunction with the redirectorrings 57 and Raschig rings 58 within the distillation column 400.

By comparison with the distillation apparatus of FIG. 4, condenser 464and the sloping tube 418 which carry overflowing condensate fromcondenser 464 replaces the reflux condenser head 60 shown at the top ofthe quartz distillation column 46 of FIG. 4. However, the distillationprocess remains the same.

In the embodiment shown in FIG. 19 the condenser 464 is similar to thatshown in FIG. 4 except for the connection to the distillation column.

There is a particularly novel safety feature which is provided in theembodiment of FIG. 19. This is the provision of an automatic computercontrolled method for quick cooling of the distillation column in anemergency. A conduit 414 which is in communication with condenser 464contains a cooled product can be sent to the distillation column 400 byopening of valve 406. This permits passage of cooled product throughconduit 404 which enters the column 400 beneath the liquid level withinthe column 400. Entry of the cooled liquid product immediately quenchesthe distillation process by reducing the temperature.

The valve 406 can be computer controlled to automatically quench boilingfor rapid system shut down. Thus, the cold liquid from the cooledproduct inventory can rapidly reduce the temperature within the column400 to below boiling temperature. This feature is particularly desirablefor the use in a glass distillation column.

Another feature of the embodiment of FIG. 19 is the elimination of thecondenser 74 and the packing column 76 as shown in FIG. 4. In theembodiment of FIG. 19, any vapor produced within the condenser whichdoes not condense is directed to the surge tank 12 through tube 412while the cooled condensed acid liquid is sent directly to the streamsplitter 4 without passing through the condenser 74 and packed column 76which are not needed.

With respect to the embodiment of FIG. 4, in operation the acid strippedof water from stripper 1 continuously enters the distillation column 46at inlet tube 54 near the bottom of the distillation column 46. Heat iscontinuously applied to the quartz tube 46 by means of the heating coils82 to effect continuous boiling of the acid. The temperature of the acidis maintained at approximately 300° C.-330° C. Boiling of the acid isaided by means of the quartz boiling rings 50 which are present toprevent excess bubbling and provide smooth boiling and convective flowand efficient heat transfer through the walls of the distillationcolumn. Boiling occurs along both sides of the boiling rings 50 so thathot acid and vapor bubbles rise along the interior walls of thedistillation column while the cooler liquid flows downwardly in thecentral area of the column.

The packing with the Raschig rings 58, the redirector rings 57, and thereflux condenser 60 act to direct the condensing acid vapors down thecentral area of the column 46 to continuously scrub the acid vaporremoving soluble and insoluble particles and other contaminantsdownwardly to produce an especially ultra highly purified acid vaporwhich continuously rises and escapes through passage 62 to the coolerand condenser 3.

The vapor which does not escape through outlet passage 62 is purified bythe effect of the repeated condensation and re-evaporation on the columnpacking 58 in conjunction with the redirector rings 57 and the refluxingliquid. The reflux head 60 augments this action by condensing a portionof the sulfuric acid vapor. The resulting liquid returns to the boilingacid in a downward direction which acts to scrub the vapor by means ofthe counter action between the upstream vapor of the volatile andparticulate impurities contained therein. Ideally, this combinationresults in a reflux of at least 50%.

By utilizing the embodiment of FIG. 19, the reflux head 60 is replacedby the overflow tube 418 and the condenser 464. In other respects theaction is the same to scrub the impurities especially insolubleparticles of less than 10 microns from the rising acid vapor.

The influx of acid and its subsequent distillation produces near thebottom of the column 46 a concentration of contaminants includingparticles which have been separated from the acid. This more highlycontaminated acid can be removed continuously as waste from the bottomof the column 46 by means of small diameter tube 48.

However, it is preferred not to remove the bottom product except afterseveral months of operation and then by shut down of the distillationprocess. It has been found that the operation of the column is soeffective that removal is needed only after a considerable period ofoperation. Moreover, by eliminating this step there is the avoidance ofthe introduction of new particle contaminators. Thus, the embodiment ofFIG. 19 does not provide for removal of waste acid.

In the cooler and condenser 3 of FIG. 4 the vapor is condensed andcooled to a process temperature in the range of about 100° C. to about150° C. A reservoir of ultrapure clean acid at a constant elevation orhead pressure is thereby produced.

It is preferred to use a relatively tall distillation column 46 to allowfor elevation of the acid without the use of a pump. In this manner, thepurified acid can be moved by means of gravitational force to the wafercleaning station 11. The need for a pump can thus be avoided and theconsequent possible contamination which can be introduced by the use ofpumps.

While the invention is described with respect to use of only fusedquartz glass, it should be understood that the invention andparticularly the distillation apparatus are not limited thereby. Theactual selection of the material for the distillation apparatus willdepend upon the type of liquid to be purified and the type ofcontaminants and particles to be removed. Thus, it is contemplated thatother materials than glass including among others stainless and otherspecialty metals can be used.

In like manner, while a distillation column is preferred as illustratedin the drawings, the invention is not limited by the actualconfiguration of the distillation chamber. Similarly, while thenucleation rings shown in the drawings are spaced from the walls, theinvention is intended to include the provision of the nucleation sitesadhered directly to the walls of the distillation chamber or be integraltherewith. The important feature is to provide nucleation sitesproximate to the walls to effect a predominant smooth, convectivecirculation pattern wherein bubbles form and rise proximate the walls ofthe distillation chamber with a downward smooth convective flow ofcooler liquid substantially centrally of the distillation chamber. Thus,different configurations of distillation chambers and different methodsof providing nucleation sites are a part of this invention if thispattern of boiling is provided.

The distillation method and apparatus of the invention are intended forremoval of dissolved impurities and insoluble particles from fluids.Thus uses other than for purification of acids such as sulfuric acid arecontemplated. The invention is not limited to the liquids to bepurified. Thus, any fluids capable of distillation are contemplated forpurification according to the invention. Also, while the distillationmethod and apparatus are shown for use in a continuous process, batchprocesses can also be used without departing from the scope of theinvention.

Using the distillation apparatus and distillation method of theinvention, it has been found that within practical limits that it isdesirable to provide a low net vapor velocity through the distillationcolumn. With a high vapor velocity, more particles are entrained. Forthe distillation apparatus shown in FIGS. 4 and 19, excellent resultshave been obtained with a total of about 700 cc of liquid sulfuric acidbeing produced per minute. The reflux ratio is about 0.2 for a total of840 cc of liquid per minute being boiled and moved through the column asvapor. This was obtained using a distillation column having a diameterof 8 inches, a height of 6 feet with 16-18 inches of liquid. Above theliquid there are 6-8 inches of free space to the packing stop, about 18inches of packing and redirector rings and the balance is free space.

THE STREAM SPLITTER

From the condenser and cooler 3 the acid stream continuously exits at 70to the wafer cleaning station 11 and to the cooler 5. This is not shownin the drawings but can comprise any convenient means. For example, twoseparate ports from the condenser and cooler 3 with valve means canconveniently accomplish this. Alternatively, a series of valves whichwill split the stream into two smaller streams can be used. Preferably,about 80% to about 98% by volume of the stream is sent directly to thesemiconductor wafer cleaner 11, while about 2% to about 20% is divertedfor electrochemical treatment. Thus, a major portion of the stream issent directly to the wafer cleaner 11 while a minor portion is processedfurther. The actual proportions of the stream are dictated by therequirements of the wafer cleaning bath. Other ranges can be used ifdesired. The exact amount should not limit the scope of the invention.The change of the purified acid liquid from sulfuric acid toperoxydisulfuric acid is effected by reaction in the electrochemicalcell 10.

THE COOLER

From the stream splitter 4, the diverted smaller stream comprisingpreferably about 2% to about 20% by volume of the stream is continuouslydirected to a cooler 5 which cools the hot process acid to approximatelyroom temperature or about 25° C. This temperature is suitable for thenext analyzing step at block 6 shown in FIG. 1.

The cooler 5 detailed in FIG. 6 is in the form of a large diameterTeflon (TM) vessel 86 having a circumferential flange 83 spaced from itstop opening. A lid 81 having a flanged top 91 fits over the top of thevessel. A plurality of bolts 85 pass through the flanged top 91 of thelid 81 and the flange 83 on the vessel 86 to hold the lid tightly on thevessel 86.

The lid 81 is provided with an inlet 90 for the introduction of acid.Two other openings 93 and 95 in the lid 81 hold the ends of a sealedsmall diameter fused quartz tube spiral 88 which is disposed withinvessel 86. The spiral tube 88 has cooling water circulating therethroughfor purposes of cooling the acid. An outlet 92 is disposed in the bottomof the vessel 86 for exit of cooled acid.

The acid from the stream splitter 4 continuously enters the cooler 5through inlet 90 where it circulates in contact with the cooling watercoils 88 and then exits at outlet 92 to the particle counter 6.

THE PARTICLE COUNTER

The particle counter 6 is a flow through device so that the entire sidestream is analyzed for particles. Other analytical instruments can alsobe incorporated at this point. Examples of such instruments which areuseful in the present process as applied to the semiconductor industry,include but are not limited to atomic absorption spectrometers for traceanalysis and laser particle counters, chromatographs, IR and UV-VISspectrophotometers; mass spectrometers; emission and plasma emissionspectrometers; electrochemical analyzers; chromatographs; and any otherdesired analytical instrument which would analyze and measure the purityof the sulfuric acid of the stream.

From the particle counter 6, the acid is continuously passed to adiluter 7 which is detailed in FIG. 7.

THE DILUTER

As shown, the diluter 7 is formed of a large diameter fused quartz glasstube 94 having a substantially centrally disposed fused quartz glasstube 96. The tube 96 has an inlet port 98 for the continuousintroduction of mete red amounts of ultrapure water which is upstreamfrom an inlet port 100 for the continuous introduction of meteredamounts of acid. An outlet 102 for central tube 96 continuously conductsdiluted sulfuric acid out of the diluter 7.

Within the quartz tube 94 in contact with the exterior surfaces ofcentral tube 96 is a water jacket of circulating cooled thermostatedwater.

The central tube 96 has disposed therein a spiral coil of small diameterfused quartz glass tubing 112 which is sealed with respect to theinterior of tube 96. The spiral coil 112 communicates with thethermostated water circulating around the exterior of tube 96 by meansof an inlet 106 and an outlet 103 through which cooling fluid such aswater circulates continuously around the tube 96 and exits at an outlet104.

In operation, a metering pump or other constant flow device not showncontinuously delivers metered amounts of ultrapure water into thecentral tube 96 through port 98. The ultrapure water is caused tocirculate through central tube 96 during which time metered amounts ofacid are continuously introduced through port 100 by means of a meteringpump 17 for the acid. The metered amounts of acid introduced into themetered amount of water are calculated to provide the desired % byweight acid/water solution.

The acid and water is mixed in the diluter 7 to provide about a 30% toabout a 60% by weight sulfuric acid/water solution. Most preferably, thesolution is comprised of 50% by weight sulfuric acid. This range ofsulfuric acid to water is governed by the requirements of the E-cell forproper operation. When the above range is exceeded in either direction,the result in the E-cell will be either no reaction and/or undesiredside reactions. Excellent results have been obtained with a sulfuricacid/water solution in the range of about 45% to about 55% by weight.

The water used must be ultrapure water. The industry standard forultrapure water is "18 megohm" which refers to its maximum resistivity.The same standard for ultrapure water is used for rinsing of the wafersafter the acid cleaning.

The process of diluting the acid causes an increase of temperature. Thetemperature is reduced by the circulating water jacket of cooling wateraround the exterior surfaces of the central tube 96 within the largediameter tube 94. Diluted sulfuric acid exiting diluter 7 is cooled to atemperature in the range of about 15° C. to about 25° C., whichtemperature is a requirement for the proper operation of the electricalcell 10.

A small portion of the diluted stream is preferably diverted to anatomic absorption spectrometer for trace analysis. This is indicated atblock 16 of FIG. 1 and is not shown in detail.

The cooled diluted acid which continuously exits diluter 7 at outlet 102is continuously passed to E-cell 10 which is detailed in FIGS. 11, 12,13, and 14.

E-CELL

As shown in FIG. 11, the electrical cell includes an anode compartment122 and a cathode compartment 124 separated by a membrane 126. The anodecompartment 122 communicates with an anolyte reservoir 120 while thecathode compartment 126 communicates with a catholyte reservoir 118.Both the anolyte reservoir 120 and the cathode reservoir 118 arepositioned above the E-cell 10.

The anode compartment 122 includes a hollow, water cooled anodeelectrode 128 having ports 130 and 132 for circulation of water in andout of the hollow electrode. A Teflon (TM) layer 134 substantiallysurrounds the anode compartment 122 for insulation purposes. It is heldin place by means of an outer plate 136. The electrode 128 also includesprojections 138 and 140 for electrical connection to a power source, notshown.

The cathode compartment 124 includes a cathode plate 142. A Teflon (TM)insulator 144 partially wraps around the E-cell in the same manner asfor the anode compartment 122. A plate 146 is held against the Teflon(TM) layer 144. The cathode plate 142 also includes electrical contacts160 and 162 which are connected to a power source, not shown.

The Teflon (TM) insulators 134 and 144 together surround the E-cell 10in the form of a box. The insulators 134 and 144 are held together bymeans of the outer plate 136 and the outer plate 146. The plates 136 and146 are bolted together at the ends thereof by means of bolts 148 and150 which are secured at the ends thereof by means of nuts 152, 154, 156and 158. This is made possible by interior threading on each of the nuts152, 154, 156 and 158 in conjunction with exterior threading on theexterior ends of the bolts 148 and 150 respectively.

The anolyte reservoir 120 is in communication with the anode compartment122 by means of a tube 164 and a tube 166. The anolyte reservoir 120 isalso provided with an outlet tube 168 for continuous discharge ofoxidant solution to the wafer bath 11. An inlet 176 is provided forcontinuous introduction of diluted acid from water cooler 9 for reactionin anode compartment 122. An outlet 180 is provided for the venting ofgases from the anolyte reservoir 120.

The catholyte reservoir 118 communicates with the cathode compartment124 by means of tube 170 and tube 172. Another tube 174 provides a meansfor draining of the catholyte reservoir 118. Catholyte solution isintroduced through inlet tube 178. An outlet 182 is provided for theventing of gases from the catholyte reservoir 118.

As shown in FIG. 13, the anode 128 is in the form of a closed, hollow,generally rectangular configuration having openings 130 and 132 for theintroduction and withdrawal of water for purposes of cooling. The anode128 also is provided with solid pins 138 and 140 for purposes ofelectrical connection.

According to a preferred embodiment, the anode 128 is composed ofplatinum plated titanium and has a wall thickness of about 100 mils. Thewater circulates only on the inside of the anode 128 with the anolytesolution circulating on the exterior of the anode electrode 128.

As shown in FIG. 14, the cathode 142 is in the form of a flat platehaving a thickness of about 20 mils. Tabs 162 and 160 are provided forelectrical connection. The cathode is preferably composed of platinumsheet although lead or graphite can be used.

The catholyte and anolyte solutions circulate on one side of theelectrodes 142 and 128 respectively between the electrodes and thesemipermeable membrane 126. The membrane 126 is preferably formed of aproprietary material manufactured by Dupont which is called "NAFION"(TM). Other materials can be used in place of the "NAFION" (TM)semipermeable membrane. Such materials must be resistant to thecorrosive action of sulfuric acid at the concentrations employed.Examples of such materials include but are not limited topolytetrafluoroethylene and silicone.

The arrangement of the catholyte and anolyte reservoirs 118 and 120spaced above the electrical cell 10 avoids the need for use of a pump incirculating the catholyte and anolyte solutions through the electricalcell. This is made possible by the use of liberated gas from waterelectrolysis chemical reactions taking place in the electrical cell 10to continuously pump the solution back to the respective reservoirs 120and 118.

In operation, the catholyte reservoir 118 is filled with catholyte whichis comprised of between about 30% and about 60% by weight sulfuric acidin water. Most preferably, 50% by weight sulfuric acid is employed.

Since this acid solution only circulates between the catholyte reservoir118 and the cathode compartment 124 of the E-cell, ultrapure sulfuricacid and ultrapure water are not required. Filling of the catholytereservoir 118 takes place through inlet tube 178.

During operation of the E-cell 10, water from the anode compartment 122continually passes through semipermeable membrane 126 into the cathodecompartment 124. This dilutes the sulfuric acid solution within thecatholyte reservoir. As a consequence, sulfuric acid must be added asneeded to maintain the desired sulfuric acid concentration.

The anolyte reservoir 120 is continuously filled with diluted sulfuricacid of approximately 30% to about 60% by weight and preferably 50% byweight sulfuric acid in ultrapure water which continuously arrives fromthe diluter 7 at a temperature of approximately 15° C. to about 25° C.This temperature range insures stable operation of the E-cell 10. At thesame time, cooling water is introduced and continuously circulatedthrough hollow anode 128 through ports 130 and 132 of E-cell 10.

With both anolyte and catholyte reservoirs 120 and 118 filled, therespective connecting anode and cathode compartments 122 and 124 fill bygravity flow.

A voltage is applied from a power source, not shown, between the anode128 (+) and the cathode 142 (-). The electrochemical reactions which arebelieved to take place in the E-cell 10 are shown below:

Anode Reaction

    Main Reaction: 2SO.sub.4 .sup.-2 →2e.sup.- +S.sub.2 O.sub.8 .sup.-2

Parasitic Reaction from Electrolysis of Water:

    2H.sub.2 O→4e.sup.- +4H.sup.+ +O.sub.2 (g)

Cathode Reaction

    2H.sup.+ +2e.sup.- →H.sub.2 (g)

This electrochemical oxidation of the diluted sulfuric acid formsperoxydisulfuric acid in the anode compartment 122. As shown above,oxygen is evolved in the process, representing a substantial volumeincrease. This oxygen gas is used to pump the oxygen and liquid in acontinuous manner up to the anolyte reservoir 120. The gas is ventedthrough vent 180. A flow meter, not shown, can be used in order tomonitor the rate of evolved oxygen which can subsequently be correlatedwith the amount of peroxydisulfuric acid generated.

The acid in the anolyte reservoir 120 with the gas removed is thenrecirculated, continuously down through the tube 164 to make continuouspasses through the anode chamber 122 of the electrical cell 10.

At the same time, a portion of the acid solution in the anolytereservoir 120 is continuously being passed to the wafer process bath 11through tube 168. The amount is equivalent to the amount which isdelivered from the diluter 7 through the inlet 176. The output ispreferably motivated by gravity flow to avoid the introduction ofimpurities which might be the case if a pump were used.

Contemporaneously, newly purified diluted sulfuric acid is continuouslyarriving from the diluter 7. In this manner, inflow from the diluter 7and outflow to the wafer bath 11 is stabilized.

If any excess liquid is produced in the anolyte reservoir 120, it can bedrained off to the surge tank 12 through port 167 for reprocessingthrough the stripper 1 and distillation column 2.

At the same time that the reaction is taking place in the anodecompartment 122, reaction is simultaneously taking place in the cathodecompartment 124. The reaction taking place here produces hydrogen gaswhich in a similar manner to the reaction in the anolyte compartment 122acts to pump solution from the cathode compartment 124 through tube 172back up to the catholyte reservoir 118. Here, the hydrogen gas is ventedaway from the catholyte reservoir 118 through vent 182.

The hydrogen thus produced can be used in any desired manner such as bycollection and further use. Alternatively, if desired, it can be dilutedwith an inert gas such as nitrogen, to render it harmless. It can thenbe vented to the atmosphere.

The catholyte solution present in the catholyte reservoir 118 which hashad the gas removed proceeds in a continuous manner down through tube170 back into the cathode compartment 124. Here, it again reacts and ispumped upwardly by means of the hydrogen gas evolved through tube 172back to the catholyte reservoir 118.

The reaction proceeds in this manner continuously. The oxidant solutionthus produced is added continuously to the wafer bath 11 together withsulfuric acid arriving continuously from the condenser and cooler 3which has been separated by means of the stream splitter 4.

Once started the E-cell operates continuously in a stable manner. Thevoltage and current will also be stable and will signal a malfunction ifthere is a change. The cell requires, as noted above, sulfuric acid inabout a 30%-60% by weight solution in water and a temperature in therange of about 15° C. to about 25° C.

The exact voltage and current will vary with the size of the cell. Theexact Figures can be arrived at empirically by testing at each currentand voltage.

The E-cell constitutes a novel feature of the invention. Here,peroxydisulfuric acid is generated by the action of the E-cell on thedilute sulfuric acid. In addition, the gases generated in the anode andcathode compartments are utilized to pump the catholyte and anolyte totheir respective reservoirs above the E-cell. This avoids the need for apump which could introduce impurities and provides a reservoir offreshly generated oxidant for addition to the wafer cleaning bath.

A preferred method of start up operation of the system is to start witha sufficient quantity of sulfuric acid solution in the range of 95% toabout 98% by weight in water. This is conveniently introduced at thesurge tank 12 and purified through the system as above described.

At the stream splitter 4, about 2%-20% by weight and preferably 2% byweight of the purified sulfuric acid is split from the main stream foroxidation in the E-cell 10. The remaining 98% to 80% by weight passesdirectly to the wafer cleaner 11.

The 2%-20% by weight purified sulfuric acid is passed to the cooler 5,particle counter 6, and diluter 7 where it is mixed with water andcooled to provide about a 30% to about 60% and preferably a 50% byweight solution of acid in water. A portion of the stream is diverted tothe atomic absorption spectrometer 16 for trace analysis. The remainderis directed to the anolyte reservoir 120.

With the catholyte reservoir 118 charged with the required amount ofsulfuric acid/water solution, the operation of the E-cell 10 is begun.Before passing the anolyte solution to the wafer cleaning station 11, aconcentration of at least about 0.05M and typically 0.5Mperoxydisulfuric acid is first reached. The wafer cleaning processrequires a concentration of least about 0.03M peroxydisulfuric acidafter mixing with the main stream of sulfuric acid to be effective. Theupper concentration limit of peroxydisulfuric acid which can be producedby this E-cell is about 1M to 2M.

When the desired molar concentration of peroxydisulfuric acid has beenreached, anolyte can be withdrawn from the anolyte reservoir and passedto the wafer cleaning station 11. At the same time additional purifieddiluted sulfuric acid is added from the cooler and condenser 3. Thesetwo streams taken together are equivalent to the amount beingcontinually withdrawn from the wafer cleaning station 11 and occasionaladditions of sulfuric acid to the surge tank 12. The additions areneeded to make up for waste acid withdrawn at the stripper 1,distillation column 2, and drag out as cleaned wafers are removed fromthe cleaning bath. By this process, a state of equilibrium isestablished which can be maintained continuously.

Occasionally, sulfuric acid needs to be added to the catholyte reservoirto counteract the dilution effect of water entering cathode compartment124 through semipermeable membrane 126 of E-cell 10. However, this aciddoes not normally contact the oxidant repurification stream. Only whenthe catholyte reservoir overflows to the surge tank does contact withthe repurification stream take place.

Sulfuric acid and peroxydisulfuric acid are highly corrosive and arevery reactive. They are capable of dissolving most metals and willoxidize. A dehydrate or sulfonate most organic compounds. Their reactionwith water generates a great deal of heat and can cause explosivespattering. For this reason, in the diluter 7 only small amounts of acidare mixed with water and extensive cooling is provided.

It is important that the apparatus of this invention utilize materialswhich are not corroded by or reactive with sulfuric acid,peroxydisulfuric acid and/or high temperatures. For this reason,standard technology vitreous quartz and polytetrafluoroethylene (TeflonTM) are preferred materials to be used in contact with the acids.

A further feature of the invention which is not shown is the monitoringof all of the separate steps. For example, liquid levels in all parts ofthe system are monitored, as well as particle content and purity for theoverall process. The latter is accomplished by means of the particlecounter 6 and atomic absorption spectrometer 16. Temperature monitoringis also provided to automatically open and close valves for consistentprocessing and for safety.

The following example is given for the purpose of illustrating theinvention and is in no way intended to constitute a limitation thereof.

EXAMPLE

Using the apparatus as above described, about 4 liters of 50% by weightsulfuric acid are added to the catholyte reservoir and about 4 liters of50% by weight sulfuric acid are added to the anolyte reservoir. Then 95%to 98% by weight sulfuric acid is added slowly to the surge tank and thepump between the fluoride removal column and the stripper is started.This causes the stripper to fill with acid which overflows to thedistillation column. Acid addition is continued until the desired levelof acid in the distillation column can be seen. At this point the systemis partially charged and about 40 liters of acid are required to reachthis point.

The main power to the system and support utilities including the oil andwater heat exchangers is switched on. The main control panel is thenturned on and all of the system diagnostics are checked. When everythingchecks out, the power to the distillation and stripper heaters areadvanced to about 20% of full power. The control panel is then observedfor the next half hour during which time the system is warmed up. Ifeverything checks out, the power is advanced to 50% of full power. Thisresults over time in visible boiling in the distillation column and theproduction of steam in the stripper.

The control panel is checked again and if all looks well, then fullpower is applied to the distillation and stripper heaters. Thetemperature is brought to and maintained at about 280° C. at thestripper. Boiling is maintained at the distillation column. As theliquid acid present in the distillation column begins to distill, theremainder of the system is gradually charged with acid. The level of thesurge tank is kept at a set level, with acid being added to maintainthat level. The amount added corresponds to the amount of acid passingthrough the distillation column. Acid additions to the surge tank arecontinued until the entire system is charged with acid. The total volumeof the system is about 60 liters.

When the system is on and fully functioning, the ultrapure water for thediluter is turned on. Then the pump for the acid to the mixer is turnedon. The valve from the splitter to the anolyte reservoir is then opened.At this point, the power to the E-cell is turned on and the E-cellbegins to operate. The time for the preceding to take place is about twohours.

When the E-cell has been operating for about two hours and the processappears to have stabilized throughout the system, the valve from theanolyte reservoir to the wafer cleaning station is opened. When thedesired operating temperature is reached at the wafer cleaning stationand the process appears to have stabilized such that input issubstantially equal to output, a sample of the wafer bath is taken foranalysis. This is conveniently done by titration.

If the desired makeup of the wafer cleaning oxidant bath is comprised of92% by weight sulfuric acid, at least 0.03M peroxydisulfuric acid, andthe balance water, then the wafer cleaning process is begun. If theconcentration of peroxydisulfuric acid is too high the flow rate intothe bath is decreased slightly. Conversely if the concentration of theperoxydisulfuric acid is too low, the flow rate into the bath isincreased slightly. This can conveniently be done by adjustment of thevalves into and out of the anolyte reservoir. For the size E-cell shown,the flow rate is about 60 ml per minute.

Initially the concentration of any trace impurities in the sulfuric acidprior to stabilization of the process is approximately 100 PPB. Afterstabilization of the process has been reached the concentration of anyspecific trace impurity is <10 PPB. Also, particles are reduced from10-100,000/cc particles of 1-15 micron size to <5/cc particles of 1micron and greater size.

During the wafer cleaning process, small amounts of acid are lost bydragout of the acid during removal of the cassettes from the wafercleaning oxidant bath. This requires periodic replacement through thesurge tank. A level sensor in the surge tank is employed to indicatewhen acid additions are required.

The system has been run for more than 1000 hours with only periodicadditions of acid. During this time, waste acid was not removed from thedistillation column or from the stripper. It is believed that, dependingupon the impurities introduced by the wafers that the system can beoperated for about three months before the concentration of traceelements would increase sufficiently high to require shut down of theprocess. At this time all of the acid is preferably discarded and a newbatch of acid added.

THE WAFER CLEANING

Semiconductor wafers are cleaned of residual particles and contaminantsacquired on their surfaces during manufacture. Cleaning of semiconductorwafers must be conducted under very clean, class 100 or better cleanroom conditions. For this reason, the wafer cleaning process isconducted in a room physically separate from the repurification process.Here, the air is highly filtered to remove all particles.

The invention is primarily directed to the method and apparatus for thereprocessing of the oxidant used in the wafer cleaning process. Thus,any wafer cleaning process station can be used without limiting theinvention.

Wafer cleaning processes are sometimes called stripping processes. Thewafers are cleaned by removing polymer and other contaminants acquiredduring their manufacture. Sometimes this includes prediffusion cleaningwhich is conducted prior to a high temperature furnace step. The sameoxidant baths are used for these processes.

Some wafer cleaning techniques utilize spraying of the wafers in orderto clean them. Another technique, which is illustrated herein, utilizesimmersion of the wafers for cleaning with a spray water rinse. Otherrinse techniques including cascade, spray only, etc are fullysatisfactory as well. The invention is not limited by the wafer cleaningprocess or by the method of rinsing. It is to the production of theoxidant bath and the use thereof in any wafer cleaning process that theinvention is directed.

As shown in FIG. 8, an acid bath 200 and a quick dump spray water bath202 are enclosed in an upright housing 210. A clear, plastic hingedcover 213 provides convenient access to the acid bath 200 and water bath202. In addition, the cover 213 guards against the introduction ofimpurities during the wafer cleaning process. The acid bath 200 and thewater bath 202 are supported within the housing 210 by a shelf 207 whichextends crosswise of the housing 210.

The wafers are normally emplaced under clean room conditions into Teflon(TM) cassette boats 212 shown in FIG. 9 which are slotted at 216 to holda row of semiconductor wafers 214. The slots 216 merely act to hold thewafers in place during immersion. FIGS. 9 and 10 show a detail of theacid bath 200.

Referring to FIG. 8, a bath 200 of oxidant solution is prepared withusually an adjacent quick dump ultrapure water bath 202 for rinsing awaythe acid. The semiconductor wafers 214, held in cassette boats 212 areimmersed first in the acid bath 200 for a time sufficient to clean themof impurities. At the end of the cleaning period, the cassette boat 212of wafers is removed from the acid bath and placed in the ultrapurewater bath 202 for rinsing.

The oxidant solution bath 200 is surrounded by an overflow trough 204.As the acid from the cooler and condenser 3 and oxidant solution fromthe anolyte reservoir 120 are continually introduced into the bath 200through inlet 206 in the bottom of the bath, the oxidant constantlyoverflows into the trough 204. From the trough 204, the acid solutionexits through outlet 208 and is passed to the surge tank 12 forrepurification according to the invention process.

The water bath 202 is shown in FIG. 8. It includes a tank 310 having asupport grid 330 spaced from the bottom to support the cassette boats212. Near the top of the tank 310 are a plurality of spray nozzles 320which spray ultrapure water down onto the cassette boats 212. A verylarge orifice drain valve 340 is disposed in the bottom of the tank 310.After the cassettes boats 212 are placed within the tank 310 on the grid330, ultrapure water is sprayed onto the cassette boats 212 from thespray nozzles 320. During this time the drain valve 340 is closed. Whenthe tank 310 is filled, an automatic sequencer, not shown, opens thelarge orifice valve 340 and quickly drains the entire tank of rinsedwater. The sequencer then shuts the drain valve and the tank 310refills. This cycle is commonly repeated five times in a period of lessthan 5 minutes to provide a totally effective rinse.

Since the acid bath 200 is constantly being renewed, cleaning of thewafers can be accomplished with a higher degree of purity than by priorart processes. This represents a considerable improvement in quality.Cost savings are also provided.

SYSTEM ARRANGEMENT

FIG. 15 shows a perspective view of a preferred arrangement of thereprocessing apparatus of the invention. As shown, a steel enclosure 228houses the bulk of the processing equipment, namely the surge tank 12,the pump 15, the fluoride removal column 14, a heat exchanger 17 notshown in the other drawing, the stripper 1 with attached condenser 13,the distillation column 2, the condenser and cooler 3, the streamsplitter 4, the water cooler 5, the particle counter 6 and atomicabsorption spectrometer 16, the pump 17, diluter 7, and E-cell 10. FIGS.16 and 17 shown an enlarged showing of the enclosure 228.

This enclosure 228 includes a pair of hinged doors 231 and 233 whichhold windows 230 and 232 respectively. Side windows 234 and 236 togetherwith windows 230 and 232 permit the viewing of fluid levels as well asthe other observable process steps. A side opening 238 permits exteriorconnections by means of tubing for pure water, nitrogen, and coolingwater. The cooling water is cooled in an adjacent heat exchanger 240.The connections are not shown. A similar opening, not shown, which islocated in the opposite side permits connection to the electricalsupplies, control systems, and monitoring system 242. By enclosing theapparatus within an enclosure, increased safety levels are ensured.

The wafer cleaning station 11 is located separately in a clean roomadjacent to the apparatus described. Only the door 246 to the clean roomis shown. Additional connections in opening 238 permits tubing, notshown, to be passed through the walls of the clean room for connectionbetween the wafer cleaning station 11 and the E-cell 10 and condenserand cooler 3, and surge tank 12 as previously described. Alternativeconfigurations, which are remote or at a different level are possiblewith non-contaminating pumping and delivery systems.

While the invention process and apparatus are discussed and illustratedusing, for example, one separator, one distillation column, and oneelectrical cell, it is contemplated that in some instances more than oneof these and other elements would desirably be used. Thus, a series ofseparators and/or a series of distillation columns could be used toincrease the purity of the sulfuric acid over that which is possiblewith only one of such elements. Likewise, in the case of the electricalcells, a series of electrical cells could be used to increase theamounts of peroxydisulfuric acid generated.

Various other modifications of the invention are contemplated which willbe apparent to those skilled in the art and which can be resorted towithout departing from the spirit and scope of the invention as definedby the following appended claims.

We claim:
 1. Distillation apparatus for the distillation of liquidscomprising;a distillation chamber having walls; means for heating saiddistillation chamber; at least one porous annular ring within saiddistillation chamber and spaced from said chamber walls to act asnucleation sites and to provide smooth convective upward flow of liquidand vapor proximate to said side walls during distillation; and, smoothconvective downward flow of liquid and vapor substantially centrally ofsaid distillation chamber during distillation.
 2. Distillation apparatusas claimed in claim 1 wherein said at least one porous annular ring islocated proximate to said walls and wherein said heating means isprovided to said walls in the vicinity of at least one porous annularring.
 3. Distillation apparatus as claimed in claim 2 wherein said atleast one porous annular ring is adhered to said chamber walls. 4.Distillation apparatus as claimed in claim 2 wherein said at least oneporous annular ring is integral with said chamber walls.
 5. Distillationapparatus as claimed in claim 1 wherein said distillation chamber has anupper portion for vapor and a lower portion for liquid and furthercomprising redirection means for conducting vapor to cause the vapor tocondense and fall substantially centrally of said distillation chamber.6. Distillation apparatus as claimed in claim 5 wherein said redirectionmeans is disposed within the upper portion of said distillation chamberand are comprised of at least one substantially truncated funnel shapedring having a larger opening directed upwardly and a smaller openingdirected downwardly.
 7. Distillation apparatus as claimed in claim 6further comprising packing disposed within said upper portion of saiddistillation chamber and further comprising reflux means forcondensation and reflux of said vapor.
 8. Distillation apparatus asclaimed in claim 7 wherein said reflux means comprises a refluxcondenser head within the upper portion of said distillation chamber. 9.Distillation apparatus as claimed in claim 8 wherein said distillationchamber, said boiling rings, said packing, said redirection rings, andsaid reflux condenser head are comprised of fused quartz glass. 10.Distillation apparatus as claimed in claim 7 wherein said reflux meanscomprises:a separate condenser means in communication with said upperportion of said distillation chamber for receipt of vapor from saidchamber and for overflow discharge of liquid condensate from saidcondenser back into said distillation chamber to provide reflux. 11.Distillation apparatus as claimed in claim 10 wherein said distillationchamber, said porous annular boiling rings, said packing means, saidredirector rings, and said separate condenser means are comprised offused quartz glass.
 12. Distillation apparatus according to claim 5further comprising liquid outlet means for removal of liquid and vaporoutlet means for exit of vapor.
 13. Distillation apparatus as claimed inclaim 1 further comprising sizing said distillation chamber with aheight sufficiently tall to enable movement of distilled liquid fromsaid distillation apparatus to a distant point of gravity flow.
 14. Adistillation apparatus for the distillation and condensation of liquidscomprising:a first elongated upright chamber having an inlet and outletfor liquids contained therein; heating means surrounding said firstchamber for purposes of heating the contents contained therein; aplurality of substantially concentric boiling rings within said firstchamber and disposed substantially axially near the bottom of said firstchamber and spaced from the walls of said first chamber; means forsecuring said boiling rings in spaced relationship to the interior wallsof said first chamber; a packing stop disposed substantially crosswiseof said first chamber above said boiling rings; at least one redirectorring within said first chamber and comprising a substantially truncatedfunnel shaped ring having a large opening and a smaller opening which isdisposed above said packing stop with its smaller opening facing thebottom of said first chamber; packing disposed above said packing stopand within and without said redirector ring; a reflux condenser headdisposed within the upper part of said first chamber; a vapor outletdisposed near the top of said chamber for continuous escape of vaporizedliquids; a condenser and cooler in communication with said vapor outletfrom said chamber; said condenser and cooler comprising a secondchamber; at least one cooling means within said condenser and cooler forcooling and condensing vapor contained therein; and, an outlet forcondensed liquid.
 15. A distillation apparatus as claimed in claim 14wherein:said condenser and cooler is disposed substantiallyhorizontally; said cooling means with said condenser is comprised of atleast one spiral tube sealed with respect to the interior contents ofsaid second chamber and having circulated therethrough cooling fluid;said condenser further including a vent for vapor which fails tocondense within the condenser; a second condenser containing coolingmeans in communication with said vent; and wherein, said refluxcondenser head is comprised of a coiled section of small diameter tubingsealed with respect to the interior of said second chamber and havingcooling fluid circulating therethrough; and wherein said packing iscomprised of small diameter tubing cut crosswise into a plurality ofsmaller lengths.
 16. A distillation apparatus comprising:a firstelongated substantially upright chamber having an inlet and outlet forliquids contained therein; heating means surrounding said first chamberfor purposes of heating the contents contained therein; a plurality ofsubstantially concentric boiling rings within said first chamber anddisposed substantially axially near the bottom of said first chamber andspaced from the sidewalls of said first chamber; means for securing saidboiling rings in spaced relationship to the interior walls of said firstchamber; a packing stop disposed substantially crosswise of said firstchamber above said boiling rings; at least one redirector ring disposedabove said packing stop; packing disposed above said packing stop andwithin and without each said redirector ring; a vapor outlet disposednear the top of said chamber for continuous escape of vaporized fluids;a condenser and cooler in communication with said vapor from saidchamber; and, a downwardly sloping conduit from said condenser andcooler to said distillation column whereby liquid condensate from saidcondenser can overflow back to said distillation column to providereflux.
 17. Distillation apparatus for the distillation and condensationof corrosive acid liquids comprising:a first distillation unit includinga chamber having walls and inlet means for introduction of feed liquidcomposed of a corrosive acid liquid to be purified and recovered andimpurities having a boiling point below that of the liquid to bepurified and solid particulate contaminant impurities of a size and sizedistribution; said first distillation unit being operative to effectseparation of the impurities having a boiling point below the liquid tobe purified and recovered by converting to a vapor substantially all ofthe impurities having a boiling point below that of the liquid to bepurified and to increase the concentration of the liquid to be purifiedand recovered over that present in the feed liquid thereby to form apartially purified liquid; said first distillation unit being composedof a material which is resistant to corrosion by the corrosive acidliquid; vapor outlet means for removal of substantially all of theimpurities having a boiling point below that of the liquid to bepurified as a vapor from said first distillation chamber; liquid outletmeans for removal of a portion of the liquid to be purified andrecovered and the solid particulate impurities therein contained in saidfirst distillation chamber; heating means for said distillation chamberfor heating the contents contained therein; said heating means beingexternal of the walls of said distillation chamber and being operativeto heat the walls thereby to heat the feed liquid to a temperature tovaporize the impurities having a boiling point below that of the liquidto be purified; a second distillation chamber having walls and liquidinlet means in communication with said liquid outlet means of said firstdistillation chamber for withdrawal of the partially purified liquidfrom said liquid outlet means of said first distillation chamber andintroduction thereof as feed liquid for said second distillationchamber; said second distillation unit being composed of a materialresistant to corrosion by said corrosive liquid material and operativeto effect separation of the solid particulate impurities from saidliquid to be purified and recovered by converting to a second vaporsubstantially all of the liquid to be purified and recovered wherebysubstantially all of the solid particulate impurities remain in anon-volatilized feed liquid contained in said second distillation unitthereby to reduce the particle size and size distribution of theparticles in said second vapor; liquid outlet means in said seconddistillation chamber for removal of at least a portion of thenon-volitized feed liquid contained therein; vapor outlet means for exitof second vapor from said second distillation chamber; means in saidsecond distillation unit composed of a material resistant to corrosionby the corrosive liquid to permit said second vapor to travel towardssaid vapor outlet means for said second vapor while retaining asubstantial portion of said particles in said second distillation unit;condensation means in communication with said vapor outlet means forcondensing second vapor to a liquid which is purified and to berecovered and which includes solid particulate contaminant impurities ofa size and size distribution less than that of the starting feed liquid;heating means for said second distillation chamber for heating thecontents contained therein; and, said heating means for said seconddistillation chamber being external to the walls of said seconddistillation chamber and being operative to heat the walls thereofthereby to heat the liquid therein to a temperature to vaporize theliquid to be purified and recovered.
 18. Distillation apparatus asclaimed in claim 17 wherein at least one of said first and seconddistillation chambers are of fused quartz glass.